Detecting trains via an independent detection system uses axle counters and/or track circuits. Detection by track circuits and axle counters are based on the principle that allows detection of trains equipped with steel wheels on steel rails. Track circuits operate when a voltage running through the steel rail energizes a track relay. Track circuits detect trains when the train axles shunt the rail voltage away from the relay, causing the relay to drop and indicating an occupied section of a guideway. Axle counters detect the passing of wheel flange by the detection head counting individual wheel axles entering into the occupied block section. For train systems where the trains do not have a steel wheel to steel rail interface (rubber tire systems, monorails, Maglev, and the like), conventional train detection systems like track circuits and axle counters are unreliable or even impossible to apply.
As an alternate to track circuits and axle counters, sometimes train mounted RFID transponders and a wayside reader are used to identify the passage of a train by reading the identifying information of a train mounted transponder. The wayside reader evaluates the passing transponder message to determine a train's movements on the guideway.
For train systems where the trains do not have a steel wheel to steel rail interface (e.g., rubber tire systems, monorails, Maglev, and the like), conventional train detection systems like track circuits and axle counters are hard or impossible to apply.
A disadvantage of the existing systems (track circuits and axle counters) is that they are connected to the rail in order to provide detection. This feature makes them susceptible to electromagnetic interference (EMI) induced into the rail. In particular, the risk for damage due to lightning strike which can cause unreliable operation and downstream equipment damage due to surge propagation of energy along the rail into galvanically coupled equipment.
Systems based on detection of transponders mounted on the train and wayside readers have the disadvantage that failure of the reader or the transponder can become dormant and hard to detect (unless special precautions are put into place) and as such might affect the safety of the system (not failsafe).